Add in-game paint HUD bound to the mesh painter

Add a code-driven HUD that appears in paint mode and binds live to the
active MeshPainter: four channel toggles, HSV pickers for albedo and
emission, sliders for metallic/smoothness/occlusion, emission intensity
and brush size. The HUD self-assembles its own overlay canvas and event
system at runtime.

Add a ModeChanged event to GameModeManager so the HUD can follow the
active mode, and guard MeshPainter against painting through the UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/MeshPainter.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using UnityEngine;
+using UnityEngine.EventSystems;
 #if ENABLE_INPUT_SYSTEM
 using UnityEngine.InputSystem;
 #endif
@@ -578,6 +579,11 @@ public partial class MeshPainter : MonoBehaviour
         if (!TryGetMouse(out bool pressed, out Vector2 screenPos))
             return;
 
+        // Don't paint through the in-game UI: clicking a HUD slider/toggle must not
+        // also stamp the mesh behind it.
+        if (pressed && IsPointerOverUi())
+            pressed = false;
+
         // Brush preview (size + mask) at the hover point - independent of painting,
         // so you see the brush before pressing the button.
         if (previewMode != BrushPreviewMode.Off && _previewMat && paintCamera)
@@ -741,6 +747,13 @@ public partial class MeshPainter : MonoBehaviour
 #endif
     }
 
+    /// <summary>True if the pointer is currently over an interactive UI element (raycast target).</summary>
+    private static bool IsPointerOverUi()
+    {
+        var es = EventSystem.current;
+        return es && es.IsPointerOverGameObject();
+    }
+
     private void OnDestroy()
     {
         if (_bakedMesh) Destroy(_bakedMesh);